Glow plugs, which have been conventionally used for parts such as a starting aid of diesel engines, include members such as a hollow cylindrical metal shell, a stick-like center shank, a heater including a heating element inside it that heats when electrified, an insulator, an external cylinder, and a clamping member. Metal glow plugs that employ a metal sheath heater as the heater and ceramic glow plugs that employ a ceramic heater as the heater have been appropriately selected and used recently, from the viewpoint of performances required by diesel engines and costs.
A ceramic glow plug generally has the following structure: A center shank is placed on the inside of a hollow metal shell with one end of the center shank protruding from the rear end. The other end of the center shank, which is near the front end of the metal shell, is provided with a ceramic heater in the shape of a round bar. The front end of the metal shell is connected to an external cylinder, which holds the ceramic heater. In the rear end of the metal shell, an insulator is inserted in a gap between the center shank and the metal shell, and a clamping member is placed at the rear end of the insulator so that the center shank is fixed.
The ceramic heater is so constructed that a heating element including a conductive ceramic is embedded in a base made of an insulating ceramic and held in it. Various studies on materials for the heating element and base that are capable of enduring use at higher temperatures have been conducted these days. For example, the employment of a material including at least one of silicides, nitrides and carbides of molybdenum and silicides, nitrides and carbides of tungsten as the main component for the heating element has been considered. On the other hand, a material including silicon nitride as its main component is known as the material for the base.
However, generally the material for the heating element is apt to have a larger thermal expansion coefficient than the material for the base. When the difference between the thermal expansion coefficient of the former and that of the latter is large, the thermal shrinkage of the former is greatly different from that of the latter during, for example, a cooling process from a heated state to a cooled state, which may cause problems such as cracks in the base due to thermal stress. As a means to make the thermal expansion coefficient of the base closer to that of the heating element is known a method in which materials with a larger thermal expansion coefficient such as metal carbides, a typical example of which is tungsten carbide, are incorporated into the material of the base. See, for example, patent documents 1 and 2.
Patent document 1 discloses a ceramic sintered body having a matrix made of a nitride ceramic and at least one substance selected from a carbide, a silicide, a nitride and a boride of a metal that has a larger thermal expansion coefficient than the matrix, wherein the ratio of the volume of the substance to that of the matrix is from not less than 1% to less than 5%; and the ceramic sintered body has a volume receptivity of 108 Ωcm or more and an insulation breaking strength at ordinary temperature of 1 kV/mm or more.
Patent document 2 discloses a ceramic heating element prepared by embedding a heating resistive body made of an inorganic conductive material in a silicon nitride sintered body including a rare earth element and silicon oxide wherein the ratio of the molar amount of the rare earth element in terms of an oxide thereof to that of silicon oxide (SiO2) converted from the amount of oxygen is from 1.0 to 2.5.    Patent document 1: JP H10-25162 A    Patent document 2: JP Patent No. 2735725
Although the method mentioned above is capable of checking cracks due to the difference between the thermal expansion coefficients, there still remain the following problems. Engines have engine oil to lubricate the contact faces of metal members and reduce friction. The engine oil may permeate into the cylinder bore due to a failure of the piston ring. This permeation may cause the engine oil to adhere to the front end of the ceramic heater, which may lead to corrosion of the base near the front end of the ceramic heater by a calcium component of the oil. A fuel air mixture and a combustion gas both including an oil component, as well as the adhesion of the engine oil, may cause corrosion. When the corrosion develops, the heating element may be exposed and the oxidation thereof may grow more serious, which may ruin the function of the glow plug.
Also, when the heater that is used for, e.g. diesel engines is repeatedly exposed to a high temperature and ordinary temperature, there is a probability that the ceramic sintered body may be cracked because of the difference between the thermal expansion of the ceramic sintered body and that of the heating element and the difference between the thermal shrinkage of the former and that of the latter, or the strength of the ceramic sintered body may be lowered by movement of metal ions in the grain boundary phases due to an exposure of the ceramic sintered body to a high temperature.
In view of these problems, ceramic heaters excellent in high-temperature properties and corrosion resistance have been demanded.